This invention relates to high-temperature, secondary electrochemical cells and batteries of such cells that can be employed as power sources for electrical automobiles, hybrid electric vehicles or for the storage of energy generated during intervals of off-peak power consumption. It is particularly applicable to electrochemical cells that employ metal sulfides as positive electrode (cathode) reactants and alkali metals as negative (anode) electrode reactants.
A substantial amount of work has been done in the development of these types of electrochemical cells and their electrodes. Various type cells showing promise have employed lithium, lithium-aluminum alloy or sodium as the reactant within the negative electrode. In the positive electrode, the chalcogens, particularly sulfur and sulfur compounds, have been used. Electrolytes of molten salt generally containing the ions of the negative reactant provide ionic conduction between the electrodes. Examples of these secondary, high-temperature cells are disclosed in U.S. Pat. Nos. 3,827,910 to Cairns et al., entitled "Homogeneous Cathode Mixtures for Secondary Electrochemical Power-Producing Cells", Aug. 6, 1974; 3,716,409 to Cairns et al., entitled "Cathodes for Secondary Electrochemical Power-Producing Cells", Feb. 13, 1973; and 3,488,221 to Hiroshi Shimotake et al., Jan. 6, 1970. A number of other pending patent applications relating to these type cells include ERDA case No. S-44,393, Ser. No. 510,840 to Yao et al. and entitled "Electrochemical Cell Assembled in Discharged State", filed Sept. 30, 1974 and ERDA case No. S-43,384, Ser. No. 416,311 entitled "Modular Electrochemical Cell", now U.S. Pat. No. 3,887,396, June 3, 1975, to Walsh et al. Each of these patents and patent applications are assigned to the assignee of the present application.
The iron sulfides FeS.sub.2 and FeS have been found to be particularly attractive sulfur compounds for use as positive electrode reactants. These materials are readily available and are much more easily contained within the cell than elemental sulfur. Although FeS.sub.2 has a lower equivalent weight and generally performs better within the positive electrode than FeS, it reacts with and degrades iron components within the cell to form FeS. Consequently, inert materials such as molybdenum or tungsten are required for use in current collectors and electrical terminals in contact with FeS.sub.2 cathode compositions. Contrastingly, FeS is relatively inert to iron and can be operated as a positive electrode reactant in contact with iron or mild steel current collectors and terminals for long periods of time.
Positive electrode compositions employing only FeS as the cathode reactant have given less than the expected performance in, for instance, reactant utilization, cell capacity and power. The addition of electrically conductive materials such as iron or carbon either as solid current collectors or as powdered materials dispersed throughout the positive electrode composition have provided only some of the desired improvements in cell performance.
One theory regarding problems in the operation of a cell employing FeS reactant is the formation during the charge cycle of a new solid phase having the tentative composition of K.sub.2 Fe.sub.7 S.sub.8 and possibly containing small amounts of Li. This composition or phase will hereafter be referred to as the J phase. The J phase contains substantial quantities of potassium from reaction with the electrolyte. Consequently, its formation causes a shift in the electrolyte composition that may have an adverse effect on the long-term operation of the cell as well as contribute to electrode swelling.
In previous studies with Li/FeS cells, metallographic and X-ray diffraction examinations of the positive electrodes have revealed that the J phase is the dominant, solid sulfide phase in a well charged cell that originally included FeS as the cathode reactant material. FeS could be produced on recharge of these cells by using somewhat higher cutoff voltages. The required charge voltage for conversion of J phase to FeS appears to vary inversely with temperature. For example, about 2 volts is required at 440.degree.C., while about 2.2 volts charge produces FeS at 380.degree.C. However, in cells employing iron current collector material, the higher cutoff voltages would tend to oxidize the iron and degrade the current collector structure.